Large format fireclay refractory bricks typically have a rectangular cross section and a top surface for contacting a tin bath and a bottom surface for not facing a tin bath. The bottom surface not facing the tin bath is supported by a steel construction in the form of a tub. Such large-format fireclay refractory bricks are manufactured in a known manner, and are formed of brick-forming material of the Al.sub.2 O.sub.3 --SiO.sub.2 system, i.e. chamotte, silika, clay,etc.
The brick-forming material typically includes a mixture of chamotte with a grain size of up to 5mm; an incombustible clay which is rich in silic acid and having a quartz component of greater than 50% with respect to the clay component with the quartz having a grain size of up to 50 .mu.m; a fine particle incombustible clay with an alkali component of up to 3%; an alkali-aluminosilicate, and; a binding agent. Additionally, one part of the chamotte can be replaced by a natural aluminosilicate, and fine particle C-glass with a particle size of less than 60 .mu.m can be used as an alkali-aluminosilicate. The brick-forming material is processed in a known manner, whereby the material is mixed and then poured into a mould. The material in the mould is shaped, such as by pressing, stamping, etc., and then allowed to dry in an atmosphere having a humidity of between 4-7%, after which the material is subsequently fired for at least 5 hours at a temperature of between 1200-1350.degree. C. After firing, the brick is cooled and then is typically ground with a grinder in order to finish the brick to a desired size and shape.
Such large-format fireclay refractory bricks used in a tub to be coated to receive the tin bath and consisting of a material of the Al.sub.2 O.sub.3 --SiO.sub.2 system are known. The here interesting part of the glass producing facility has a steel construction being coated with the large-format refractory bricks. Thereby a production tub is formed being filled with liquid tin. The melted glass is poured out on the surface of the tin bath. The glass expands on the surface of the tin bath and is drawn as a thin band with a substantial width over the surface of the tin bath. This is the known way of manufacturing flat glass according to the float technique. Manufacturing flat glass temperatures of about 600.degree. C. to 1200.degree. C. occur above the surface of the tin bath. Manufacturing special kinds of glass even higher temperatures may occur. Due to these higher temperatures the large-format fireclay refractory bricks at all of its different points are subject to higher temperatures. It is known to cool the tub of the steel construction containing the bricks and the tin bath from outside in order to freeze the liquid tin in the gaps between the adjacent side surfaces of the bricks and thereby prevent the passage of liquid tin to the outside.
During the manufacturing of flat glass Na.sub.2 O diffuses into the liquid tin so that sodium and atomic oxygen is solved in the tin bath. An exchange of sodium happens between the tin and the material of the fireclay refractory brick. This finally may lead to chipping off of layers of the top surface of the blocks facing the tin bath. In addition, there is the danger of penetration of liquid tin in the gaps between the side surfaces of adjacent bricks or of a passage of liquid tin through the gaps to the outside respectively. This is prevented by cooling as mentioned above. The cooling must be increased if above the tin bath higher temperatures occur than about 1000.degree. C. Cooling the tub there is the danger that the cooling effect is not uniform at all points of the tub. This is especially the case if the cooling should fail. In both circumstances the temperature of the tin bath varies which results in a minor quality of the produced glass.
The known fireclay refractory bricks have an open porosity, i.e. a percent by volume of the open pores extending to the outside surface of the brick, which generally is in the amount of 20 to 25 percent by volume. These known blocks have a closed porosity also, i.e. a percent by volume of closed pores in the interior of the block. The percentage of the open porosity and the closed porosity are similar with respect to the known bricks.